The decoders of subscribers of Pay-TV contain a decrypting unit able to treat the arriving signals by cable or by hertz. These signals can be analog or digital.
These signals are of different types, according to whether they contain audio type information, video or of control.
The latter category includes management messages (called EMM messages) that is to say messages containing controls directed to a decoder or to a group of decoders, and control messages (called ECM messages), that is to say containing authorization messages among others, information allowing to decrypt the signals in transmission.